Screen printing is a big business that continues to grow with the technology being used on tee shirts, caps, sleeves, bumper stickers and other items. One type of screen printing apparatus is where a multiplicity of screens are rotated about a common point to be placed over a plurality of work holders. The screens are rotated while in an elevated or raised position and then when they are over a workpiece to be printed are lowered onto the work. A number of colors are frequently printed on the same workpiece and each color must be registered with the others to give a satisfactory product. When only one screen is lowered at a time, registration with the workpiece is relatively easy. But if a multiplicity of screens are to be lowered at the same time, referred to as simultaneous printing or "all screens down" printing, registration at one work station may cause misregistration at a second work station because of imperfections in the equal spacing between the screens. Even if such spacing is set correctly at the factory, the machines will lose their initial settings in actual use. These problems were recognized in U.S. Pat. No. 4,974,508 issued Dec. 4, 1990 to the same inventors as the present invention. U.S. Pat. No. 4,974,508 is referred to and incorporated herein by reference.
The apparatus shown in U.S. Pat. No. 4,974,508 greatly improved the previous screen printing apparatus and permitted all screen heads to be lowered and acceptably registered for simultaneous printing and greatly increased the productivity of the machine. Although all the heads could be used to print simultaneously, frequently only several heads are so used at one time but even then there is significant improvement over the normal apparatus.
Even though the apparatus shown in U.S. Pat. No. in 4,974,508, as it relates to the "all screens down" feature, was a substantial improvement over prior apparatus, it was still found that the disc required a division into precise angular locations. For example, in a six color machine the disc is normally 40 inches in diameter so the angles are divided on a 20 inch radius. In such a machine the print may be at a 60 inch radius so the registration point at 20 inches to cause the prints to register at 60 inches has a multiplier of three so even if the original registration at 20 inch was set up with a 0.002 inch tolerance the registration at 60 inches would be 0.006 inches and that sometimes is not completely satisfactory.